Image forming apparatuses capable of forming a color image on a sheet generally include photosensitive members for yellow, magenta, cyan and black, and developing rollers for supplying color toners to the corresponding photosensitive members. In image formation, the toners are respectively supplied from the developing rollers to the photosensitive members, whereby toner images of the respective colors are formed substantially at the same time. In the case of a direct transfer system, the color toner images are transferred from the respective photosensitive members onto a sheet in superposed relation, whereby the formation of the color image on the sheet is achieved. In the case of an intermediate transfer system, the color toner images are once transferred from the respective photosensitive members onto an intermediate transfer belt in superposed relation to form a color toner image on the intermediate transfer belt, and the color toner image is further transferred from the intermediate transfer belt onto a sheet, whereby the formation of the color image on the sheet is achieved.
The toners are each partly untransferred onto the sheet or the intermediate transfer belt, and remain on the corresponding photosensitive member. Therefore, a recovery roller is provided in association with the photosensitive member for recovering the toner remaining on the photosensitive member, and a predetermined recovery bias is applied to the recovery roller. The toner remaining on the photosensitive member is transferred onto the recovery roller from the photosensitive member by static electricity when being brought into opposed relation to the recovery roller.
The toner transferred onto the recovery roller is electrostatically adsorbed on the recovery roller to be accumulated on the recovery roller. If the toner is accumulated in an increased amount on the recovery roller, the recovery roller fails to electrostatically adsorb all the transferred toner, and some of the toner is released (returned) from the recovery roller onto the photosensitive member. Even if the toner is accumulated in a smaller amount on the recovery roller, a very small amount of the toner is released from the recovery roller onto the photosensitive member.
Since a bias is applied to the developing roller, the toner released from the recovery roller onto the photosensitive member is transferred onto the developing roller when being brought into opposed relation to the developing roller with the developing roller kept in contact with the photosensitive member. Therefore, even if the toner is released from the recovery roller onto the photosensitive member, there is no possibility that the toner is transferred onto the sheet or the intermediate transfer belt.
However, if the developing roller is spaced from the photosensitive member, the toner released from the recovery roller onto the photosensitive member is not transferred onto the developing roller, but transferred onto the sheet or the intermediate transfer belt. For example, some of the color image forming apparatuses are constructed such that, when a monochrome image is to be formed on a sheet, the yellow, magenta and cyan photosensitive members are kept apart from the corresponding developing rollers. In such image forming apparatuses, if the toners are released onto the yellow, magenta and cyan photosensitive members from the corresponding recovery rollers during the formation of the monochrome image, the released toner is disadvantageously transferred onto the sheet formed with the monochrome image.